fix(errors): Report errors on unknown #[serde(...)] attributes

Closes #51, #175, and #187

serde_tests/tests/token.rs

@@ -379,7 +379,6 @@ impl<I> de::Deserializer for Deserializer<I>
     fn visit<V>(&mut self, mut visitor: V) -> Result<V::Value, Error>
         where V: de::Visitor,
     {
-        println!("visit {:?}", self.tokens.peek());
         match self.tokens.next() {
             Some(Token::Bool(v)) => visitor.visit_bool(v),
             Some(Token::Isize(v)) => visitor.visit_isize(v),